This invention claims priority of a German patent application DE 100 16 377.7 which is incorporated by reference herein.
The present invention concerns an apparatus for combining light from at least two laser light sources. Moreover the invention relates to a confocal scanning microscope which has an apparatus for combining light.
Apparatuses of the generic type have been known for some time from practical use, and are utilized principally in cases where light of different wavelengths from several laser light sources is combined into one light beam. In confocal scanning microscopy in particular, it is necessary to combine light from several laser light sources of different wavelengths into one common coaxially proceeding light beam, so as thereby to illuminate the same specimen point with light of the different wavelengths. If the light beams are not combined in exactly coaxial fashion, the undesirable result is several illumination foci at different specimen points.
DE 196 33 185 discloses, per se, a polychromatic point light source for a scanning microscope which has a beam combiner that coaxially combines the light from several laser light sources of different emission wavelengths, the beam combiner being configured as a monolithic unit.
Laser light of different wavelengths from several lasers is usually combined using so-called dichroic beam splitters. These are transparent beam splitter plates which have a coating that possesses a different transmission or emission characteristic as a function of the respective wavelength of the light.
In confocal scanning microscopy, gas lasers or mixed gas lasers whose emission light has wavelengths that are suitable for exciting fluorescent dyes are principally used to illuminate a specimen. Semiconductor lasers or solid-state lasers have hitherto seldom been used in confocal scanning microscopy, although they are considerably more economical than gas lasers in terms of acquisition price. The reason for this is the low output power of semiconductor or solid-state lasers, typically in the range of a few mW. Low-cost helium-neon lasers could also be used at some of the wavelengths of interest for confocal scanning microscopy if their output power were sufficient for the purpose.
It is therefore the object of the present invention provide a laser light source with an increased power output at a reasonable price.
The above object is achieved by an apparatus for combining light, which comprises at least two laser light sources, each of which defining a light beam wherein the light from the laser light sources has at least approximately the same wavelength; and that at least one beam combining unit which combines the light beams at least largely lossless, wherein the combination of the light beams is accomplished with reference to at least one characteristic property of the light beams.
It is a further object of the present invention to make laser light sources of low output power usable as light sources, in particular for a confocal scanning microscope.
The above object is achieved by a confocal scanning microscope which comprises: at least two laser light sources, each of which defining a light beam wherein the light from the laser light sources has at least approximately the same wavelength; and that at least one beam combining unit which combines the light beams at least largely lossless, wherein the combination of the light beams is accomplished with reference to at least one characteristic property of the light beams.
What has been recognized firstly according to the present invention is that it is not necessary to dispense with the use of economical laser light sources having only low output power if it is possible to combine their light beams in at least largely lossless fashion. The multiple combining of laser light sources of low output power can result in an output power which corresponds to that of one conventional laser, so that the use of a conventional laser having an output sufficient for confocal scanning microscopy can be omitted. The complex and vibration-sensitive air- or water-cooling system of such a laser is thus also, advantageously, not necessary, resulting in a simplified laboratory infrastructure and, in particular, eliminating the irritating noise level of an air cooling system.
In very general terms, beam combination is accomplished with reference to at least one characteristic property of the light beams. A xe2x80x9ccharacteristic propertyxe2x80x9d of the light beams is to be understood in this context as, for example, the polarization.
In the context of confocal scanning microscopy in particular, it is necessary for the combined light from several laser light sources to proceed exactly coaxially, since the several light sources then have a single common illumination focus.
In terms of the dimensioning of the beam combining unit, it is very advantageous if the light beams proceed in collimated fashion. As a result, the beam cross section of the beam path is the same at all points in the beam combining unit, so that as compared to a divergent beam path, a compact design is possible.
In a concrete embodiment, linearly polarized light from two laser light sources is combined together. The light of most lasers is in any case linearly polarized, so that no further actions are necessary in order to utilize the advantages resulting therefrom, for example a small number of optical components.
Four different characteristic properties of the light, on the basis of which the beam combination according to the present invention is performed, are discussed below. These are:
the polarization of the light;
the phase of the light;
the pulse profile over time of the light; and
the identical numerical aperture of a glass fiber.
In a concrete embodiment, light combination on the basis of polarization as the characteristic property of the light could be performed with the aid of a polarization beam splitter. A Glan-Thompson prism is preferably suitable for this. The polarization beam splitter preferably combines light beams whose polarization directions are substantially perpendicular to one another.
The polarization direction of the light from the one laser light source is set in such a way that it is deflected by the polarization beam splitter. The polarization direction of the light from the other laser light source is set in such a way that it passes through the polarization beam splitter. Assuming a suitable relative arrangement of the light beams that are to be combined, the result is a combined, coaxially proceeding light beam from the two laser light sources.
In an alternative embodiment, a polarization beam splitter and a Faraday rotator are arranged between two light beams from two laser light sources proceeding coaxially with one another in opposite directions. The polarization direction of the light from the first laser light source is set in such a way that it passes through the polarization beam splitter. The polarization direction of the second laser light source is set in such a way that after passing through the Faraday rotator arranged after the polarization beam splitter, it is at least largely parallel to the polarization direction of the light from the first laser light source. The light from the two laser light sources accordingly has the same polarization direction, specifically between the Faraday rotator and the second laser. The light from the first light source can penetrate into the second laser if the wavelength of the first laser light source conforms to the resonant wavelength of the resonator of the second laser. If the resonance condition is not met, the light from the first laser light source is for the most part reflected at the coupling-out mirror of the second laser light source. In both cases, the light from the two laser light sources now proceeds coaxially in the same direction, assuming suitable alignment of the optical components.
The Faraday rotator is configured in such a way that it rotates the polarization direction of a laser beam substantially 45xc2x0. The Faraday rotator thus rotates the polarization direction of the light from the first laser light source 45xc2x0 after it has passed through the polarization beam splitter, and conforms to the polarization direction of the second laser. After reflection of the laser light from the first laser light source at a mirror of the second laser light source, the two light beams, now coaxially combined, propagate in the direction of the Faraday rotator, which rotates the polarization direction a further 45xc2x0 as they pass so that the polarization direction of the light beam from the first laser light source is substantially perpendicular to the polarization direction of the two coaxially combined light beams. The polarization beam splitter, preferably configured as a Glan-Thompson prism, now deflects the two combined light beams so that the deflected, coaxially proceeding, combined light beams can be used for illumination in the confocal scanning microscope.
A fiber Y-coupler could be provided as the beam combining unit. In this context, in order to combine the light beams from two laser light sources, the polarization direction of the light from the one laser light source must be set in such a way that the light of the non-continuous glass fiber of the fiber Y-coupler is coupled into the continuous glass fiber at the coupling point. The polarization direction of the light from the other laser light source must be set in such a way that the light at the coupling point remains in the continuous glass fiber of the fiber Y-coupler. The light emerging from the continuous glass fiber can be used to illuminate a confocal scanning microscope. Preferably, a polarizing fiber Y-coupler is used as the beam combining unit. This fiber Y-coupler comprises polarizing glass fibers which allow the coupled-in light to be transmitted in almost lossless fashion, and in that context linearly polarize the light. With a polarizing fiber Y-coupler, an exact adjustment of the polarization direction of the light from the two laser light sources would therefore advantageously not be necessary.
A double-refracting optical element or an acousto-optical tunable filter (AOTF) could furthermore be provided as the beam combining unit. In this context, the polarization direction of the light from the first laser light source must be set in such a way that it at least largely conforms to the polarization direction of the extraordinary beam of the beam combining unit. The polarization direction of the light from the second laser light source must be set in such a way that it at least largely conforms to the polarization direction of the extraordinary beam of the beam combining unit. In this fashion, beam combination can again be accomplished in almost lossless fashion utilizing the double refraction effect.
Beam combination could be based on the characteristic property of the phase of the light of the light beams that are to be combined. In this context, beam combination is accomplished in accordance with the time reversal of a beam division at an interface or at a beam splitter plate. The xe2x80x9ctime reversal of a beam division at an interfacexe2x80x9d is to be understood in this context to mean that two light beams coming from different directions can be combined at an interface into one single light beam if both light beams have exactly the same wavelength and polarization direction, and moreover have exactly the same phase relationship with one another. Then and only then can the two light beams interfere constructively and ultimately be combined into one light beam. The reason is that the light beam that is to be reflected at the beam splitter plate then has no component which is transmitted through the beam splitter plate.
For this purpose, provision is made in particular for the light from the further laser light sources to be combined with beam splitter plates. For largely lossless beam combination of the light from several laser light sources, it is necessary for the light beams from the laser light sources that are to combined to have a well-defined phase relationship. A phase relationship or phase equalization of this kind between several laser light sources can be achieved by corresponding synchronization of the laser light sources. For phase equalization, light from a first light source is first divided into several partial beams. The divided partial beams are then respectively coupled into the further laser light sources. The coupling of a partial beam into one of the laser light sources can be accomplished at any mirror of that laser light source.
To prevent any feedback of light into a laser light source, an optical diode is placed before or after it. The optical diode is preferably embodied as a Faraday rotator. The use of a Faraday rotator in conjunction with a Glan-Thompson prism or an acousto-optical modulator (AOM) or an optical circulator is also conceivable.
One important prerequisite for this beam combination is that the laser light sources have a coherence length that is at least of the order of magnitude of the physical dimensions of the beam combining apparatus. Phase matching of the individual laser light sources is also provided for. Phase matching could be accomplished, for example, with two wedge-shaped optical components placed together. These two components could be displaced with respect to one another transversely to the optical axis, the thickness of the resulting plate thereby being adjustable. Depending on the wedge angle between the two components, the thickness of the resulting plate can be varied very sensitively. As a result, the optical path of the light that passes through the optical component can be varied relative to the other light beams, so that the phase of that light beam can be matched. A phase matching means of this kind could be placed before or after each laser light source. Whether and where it is to be provided depends on the concrete implementation of the overall beam path. Alternatively, phase matching could be performed by displacing the laser light sources together with the beam splitter plates or mirrors associated with them.
In the case of pulsed laser light sources, the pulse profile over time could be provided as a further characteristic property for combining the light beams.
An acousto-optical deflector (AOD) or electro-optical deflector (EOD) is provided as the beam combining unit for this purpose.
The pulsed laser light sources emit light pulses synchronously with one another. The pulses of the laser light sources are offset in time with respect to one another. Synchronization of the pulsed laser light sources could be achieved by the fact that the pulse-train frequency of each laser light source is in almost exact conformity. A corresponding matching between the time offsets of the laser light sources can be accomplished by inserting optical elements into the respective partial beam path, the optical elements each having a different optical path and thus a different transit time.
The light beams from the pulsed laser light sources strike the beam combining unit from different directions. The individual light pulses are deflected, by a corresponding activation of the AOD or EOD, in the direction of a coaxially proceeding light beam. In this context, the activation of the beam combining unit, embodied as an AOD or EOD, is synchronized with the pulse train of the laser light sources.
Beam combination could be based on the characteristic property of the identical numerical aperture of a glass fiber. For this purpose, the glass fiber is preferably embodied as a single-mode fiber. For beam combination, light from at least two laser light sources is focused onto one end of a glass fiber. The goal in this context is for almost the entire light intensity of each laser light source to be focused into the entrance aperture of the glass fiber.
If the required light output of two combined light beams is not sufficient for adequate illumination of a specimen, cascaded beam combination of several laser light sources is provided for. In very general terms, the combined light beam from two laser light sources is combined with a further light beam from a third laser light source. Ultimately the process of combining two input light beams into one output light beam can be performed as often as desired and in any manner desired, so that the available light output can be scaled.
In particularly advantageous fashion, polarizing glass fibers are used for this purpose. Light in any desired polarization state has a linear polarization after passing through a polarizing glass fiber of this kind. Polarizing glass fibers are similar in construction to polarization-retaining glass fibers, and have become commercially available.
The combined light from at least two laser light sources generally has two polarization directions perpendicular to one another. This combined light could be coupled into a polarizing glass fiber, which would result in a linear polarization. The light that emerges from the glass fiber could then be combined with at least one further light beam; the further light beam could also be the result of combining two laser light sources.
Alternatively or in addition thereto, cascading of several polarizing fiber Y-couplers is provided for.
Cascaded beam combination is also conceivable in the embodiment having two light beams from two laser light sources proceeding coaxially in opposite directions from one another, and having a polarization beam splitter and Faraday rotator. For this purpose, a polarization beam splitter and a Faraday rotator are placed after the combined light from the two first laser light sources. A light beam from a third laser light source proceeds in the opposite direction, coaxially with the combined light beam from the two first laser light sources. The polarization direction of the third laser light source is set in such a way that it is at least largely parallel to the polarization direction of the combined light from the first two laser light sources after passing through the second Faraday rotator located after the second polarization beam splitter. The light from the third laser light source, together with the light from the first two laser light sources reflected at a mirror of the third laser light source, is deflected by the polarization beam splitter so that the now-combined light of the three laser light sources can be used as illumination for a confocal scanning microscope.
In all the embodiments, the light that is to be combined is light from identical lasers and/or light from at least similar lasers and/or light from lasers of different types.